This invention relates to a circuit for driving the base of a transistor, and more particularly to a base driving circuit which is small in size and inexpensive to manufacture and will produce a continuous base drive current stably for a power transistor used as an on-off switch for an inverter, for example.
If a power transistor is used as an on-off switch in PWM (pulse width modulation) type inverter, the drive of this power transistor is different from a thyristor operated only by a gate signal, and requires supply of continuous current to the base during conducting (ON). At time of cutoff (OFF), it is required to apply to the base a current in the opposite direction to that during conducting (ON) in order to turn off the power transistor quickly. Thus, in order to cause the power transistor to conduct, it is necessary to supply a continuous current to the base. If a main circuit (for instance, an inverter) including a power transistor is not insulated from a control circuit for driving the base, such a requirement can be easily fulfilled.
In case that the main circuit has to be insulated from the base drive circuit, it is proposed that a pulse transformer is mounted in a base drive circuit. But, such a system requires a high reliability, because it must perform an important role for insulating a signal generated from a control circuit and transmitting it to the base of a power transistor assuredly to drive the base. Since the power transistor can be reduced to low price by the advance of manufacturing technique with the rapid advance to large capacity and high efficiency, a base drive circuit of this kind is also required to be small-sized and inexpensive.
As described before, it is necessary to supply a continuous base current to the power transistor of the main circuit during conducting (ON). FIG. 1 shows a base drive circuit according to a prior art for accomplishing the above. FIG. 2 is a schematic illustration of the waveforms of signals appearing at predetermined portions of the above base drive circuit. The base drive circuit shown in FIG. 1 uses two pulse transformers for ON drive of the base. That is, ON signal of ON-OFF command signals (ref. FIG. 2) obtained in a control circuit in the preceding stage is divided into two high-frequency pulse string signals (A.sub.1, B.sub.1), which are respectively added to each base of transistors TR.sub.1, TR.sub.2 for driving a pulse transformer. ON signals appearing at secondary side of ON pulse transformers T.sub.1, T.sub.2 are rectified by diodes D.sub.1, D.sub.2 and supplied to the base of a power transistor TR3 as a base current insulated from the control circuit to thereby drive the base. If the power transistor TR.sub.3 is cut off (OFF), an OFF pulse transformer T.sub.3 is driven by a transistor TR.sub.4, and a counter bias is applied to the base of the power transistor TR.sub.3 to turn off the power transistor rapidly.
In the base drive circuit of this prior art, when only one of a series of high-frequency pulse string signals is used, the base current is an interrupted wave. So two pulse transformers are used for ON signals to obtain a continuous base current. But in this case, there is a disadvantage of a plural number of pulse transformers resulting in not reduceing the drive portion. Since it adopts a system of dividing a command signal into two high-frequency pulse string signals, insulating same from a control circuit through a pulse transformer, and rectifying same at secondary side by a diode, it has a disadvantage of causing intermissions in the base current after rectification due to a delay time at time of rise of the pulse transformer. Such intermissions of the base current may cause a collector current of the power transistor TR.sub.3 to break. Then, it is proposed to inset signal extending means for giving an overlap period between two pulse string signals so as to supply a base current having no breaks while ON signals are added thereto. But this proposal has a difficulty that a control circuit becomes complicated, going against the direction of lowering the price.